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To Asa Gray 18 February [1860]1

I received about a week ago two sheets of your Review; read them, & sent them to Hooker; they are now returned & reread with care, & tomorrow I send them to Lyell.—2

Your Review seems to me admirable; by far the best which I have read.— I thank you from my heart both for myself, but far more for subject-sake. How curious your contrast between the views of Agassiz & such as mine is very curious & instructive.3 By the way if Agassiz writes anything on subject, I hope you will tell me.— I am charmed with your metaphor of the streamlet never running against the force of gravitation.4

Your distinction between an hypothesis & theory seems to me very ingenious; but I do not think it is ever followed.—5 Everyone now speaks of the undulatory theory of light; yet the ether is itself hypothetical & the undulations are inferred only from explaining the phenomena of light.— Even in the theory of gravitation, is the attractive power in any way known, except by explaining the fall of the apple & the movements of the Planets? It seems to me that an hypothesis is developed into a theory solely by explaining an ample lot of facts.

Again & again I thank you for your generous aid in discussing a view, about which you very properly hold yourself unbiassed.—

My dear Gray | Yours most sincerely | C. Darwin

There is Review in last Annals & Mag. of N.H. on my Book— rather bitter: I feel moral certainty it is by my good friend the Entomologist Wollaston: I have not derived any new idea from this Review.—6 Several Clergymen go far with me.— Rev. L. Jenyns, a very good naturalist: Henslow will go a very little way with me & is not shocked at me; he has just been visiting me.

Writing of artificial selection, Gray stated that ‘man leads the course of variation as he leads a streamlet,—apparently at will, but never against the force of gravitation,—to a long distance from its source, and makes it more subservient to his use or fancy.’ ([Gray] 1860a, p. 166). The phrase is marked in CD’s copy (Darwin Pamphlet Collection–CUL).

Gray distinguished two ways in which CD’s theory might be substantiated ([Gray] 1860a, p. 162): Either, 1, to assign real and adequate causes, the natural or necessary result of which must be to produce the present diversity of species and their actual relations; or 2, to show the general conformity of the whole body of facts to such assumption, and also to adduce instances explicable by it and inexplicable by the received view,— so perhaps winning our assent to the doctrine, even though the cause of the assumed variation remain as occult as that of the transformation of tadpoles into frogs … The first line of proof … would establish derivation as a true physical theory; the second, as a sufficient hypothesis. Gray made this distinction prior to discussing the nature of CD’s theory and its strengths and weaknesses.